The present invention relates to a method of upgrading a plant for transmitting electric power through High Voltage Direct Current (HVDC), said plant comprising two converter stations interconnected by a pole conductor of a monopolar direct voltage network and each having an alternating voltage side for feeding electric power from one of said alternating voltage sides to the other, each station having a line commutated Current Source Converter having at least one phase leg with one end connected to said pole conductor of the direct voltage network and the other end having an earth connection in the form of a connection to an earth electrode or a return current conductor, as well as such an upgraded plant for transmitting electric power.
The invention is not restricted to any particular levels of powers transmitted on said direct voltage network or voltages thereof with respect to ground, but said voltages are above 1 kV and often above 100 kV.
The alternating voltage sides of the two stations are typically two alternating voltage networks, but any other type of alternating voltage side is conceivable, such as a connection to a generator of a power generating plant.
A plant according to the introduction is shown in appended FIG. 1, in which 1 and 2 are the two converter stations having each a line commutated Current Source Converter 3, 4 each having an alternating voltage side 5, 6 in the form of an alternating voltage network. The two stations are interconnected by a monopolar direct voltage network having a pole conductor 7, which may be an overhead line or a cable. The opposite ends of the converters have each an earth connection by being connected to an earth electrode 8, 9. However, this earth connection may just as well be a connection to a return current conductor on ground potential extending between the stations 1, 2 for conducting a return current therebetween. The current I may in such a plant only flow in one direction in said pole conductor from one station 1 to the other 2 and will then return through the earth electrodes or a said return current conductor, often in the form of a cable. Change of power feeding direction will take place by changing the polarity of the pole conductor. Governmental permissions are to be obtained for the electrode current flowing into the ground where the stations are located.
There are incentives for upgrading such a plant. “Upgrading” is to be interpreted as making constructional modifications improving any property of the operation or behaviour of such a plant.
A disadvantage of a plant of the type shown in FIG. 1 is that the power transmission therethrough will be unbalanced, which may cause disturbances.
WO2007/073256 discloses an HVDC network comprising two converter stations connected to each other by a first and a second transmission conductor. Each converter station comprises a line-commutated converter, i.e., a current source converter, and a switching arrangement capable of directing the flow of current through either conductor such that the potential of each conductor is the same irrespective of the effective transmission direction. The current flow is alternated between the first and the second conductor when the effective power transmission changes direction and the voltage is zero.
Qahrman and Gole (A VSC based series hybrid conveter for HVDC transmission, in Proceedings Canadian conference on electrical and computer engineering, 1 May 2005, pages 458-461,XCP002531709) discuss upgrading a conventional HVDC converter by adding a voltage source converter in series with an existing line commutated converter, resulting in a hybrid converter and a control system for the hybrid converter.